Method and system for improving accuracy in autorefraction measurements by including measurement distance between the photorecptors and the scattering location in an eye

ABSTRACT

A method and associated system improve accuracy in objective refraction measurements by including the measured distance between the photoreceptors of a subject&#39;s eye and the scattering location of light during the objective refraction measurements. Chromatic aberrations in the objective measurements are also compensated. The distance between the photoreceptors and the scattering location may be determined by adjusting a distance between a rotating speckled light pattern and an eye until the speckled light pattern appears to be stationary, or by employing a Scheiner disk.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/US03/20187 filed on 27 Jun. 2003, claiming the priority benefitunder 35 U.S.C. § 119(e) from U.S. Provisional Patent Application No.60/391,668 filed on 27 Jun. 2002, and from U.S. Provisional PatentApplication No. 60/403,362, filed on 15 Aug. 2002, the entirety of eachof which is hereby incorporated by reference for all purposes as iffully set forth herein.

BACKGROUND AND SUMMARY OF THE INVENTION

1. Field of the Invention

This invention pertains to the field of subjective measurements andcharacterizations of an eye, and more particularly, to measurements andcharacterizations of an eye with wavefront analysis devices.

2. Description

FIG. 5 is a diagram of a human eye 500, illustrating the choroid 510,the fovea layer 520, the lens 530 and the cornea 540. The fovea layer520 is the area of the retina that contains the densest concentration ofphotoreceptors.

The location of the photoreceptors in the human eye is difficult todetermine accurately relative to other structures in the retina. Thephotoreceptors absorb visible light and they are transparent to infraredlight, making an accurate measurement of their location difficult.

However, there are a number of contexts where an accurate determinationof the location of the photoreceptors would prove very beneficial. Inparticular, as explained in more detail below, it would be beneficial toprovide a system and method for accurately measuring the choroid tophotoreceptor distance in the eye.

The retina requires a constant supply of blood for it to remain healthy,and in fact consumes the greatest amount of oxygen (per weight) of anytissue in the human body. Evidence from several sources show that thephotoreceptor layer ranges from 0.1 mm to 0.4 mm from the choroidalblood supply in normal human eyes. The average distance between thechoroid and the cones in the fovea is 0.2 mm. Although this distance hasnot been well studied before, it seems reasonable to suspect that insome individuals an abnormally large distance from the nourishingchoroid to the photoreceptors may predispose them to visual defectsresulting from sight degrading diseases such as glaucoma and diabetes.Early identification of abnormal photoreceptor to choroid spacing mayresult in improved patient health.

The retina is supplied with blood by two means. The inner two-thirds isnourished by branches from retinal vessels on the inner surface of theretina, while the outer one-third is nourished by the choroid. However,the fovea centralis is nourished solely by the choroid. Evidently, anyoverlying retinal vessels in the region of the fovea would block lightfrom reaching the photoreceptors and would result in greatly reducedvisual acuity. Since the choroid is the sole nourishment of the fovea,any abnormalities in the choroidal blood supply could result in reduceddiffusion of oxygen and nutrients into the fovea and that might causecell damage and reduced vision. While a large spacing would not normallyresult in visual disfunction, if the patient were to develop diabetes orglaucoma, the patient could experience an unusually rapid progress indecline of visual function. Such patients would need to be monitoredmore closely. Similarly, some populations, such as Native Americans, arenormally more closely monitored for the onset of diabetes. It may bethat data concerning choroid to photoreceptor spacing would indicatethat only a small subset of those populations are actually in need ofclose monitoring and examination intervals could be increased for theremainder of those populations.

A second application of this measurement of the choroid to photoreceptordistance would be to improve the accuracy of objective refractivemeasurements that are used to determine a patient's eyeglassprescription. Many commercially available autorefractors and wavefrontaberrometers reflect light off of the choroid (or the sclera) to providea light source for such an automatic measurement. Technicians ratherthan highly trained doctors can perform autorefractions, and theautorefractions are faster to perform than subjective refractions.

Most people are familiar with the process of subjective refraction whenan ophthalmologist flips lenses of different strengths in and out of thepatient's field of view and asks the patient if a letter on the walllooks clearer or fuzzier. The repeatability of subjective refraction isgenerally considered to be about +/−0.25 Diopters. A few practitionerswith better skill and more time to spend with the patient can achieverepeatabilities of +/−0.12 diopters.

Meanwhile, the repeatability of autorefraction measurements is betterthan +/−0.1 Diopters for almost all modem autorefractors, and thatrepeatability is much better than that of subjective refraction

However, according to several review articles, about 20% of patientswill have differences between a subjective refraction and anautorefraction greater than 0.5 Diopters. Significantly, repeatedautorefractions performed on a particular patient from the 20% groupwill consistently give the same disagreement with subjective refraction,so clearly there is some kind of structural difference in those patientseyes relative to the general population.

Eyeglasses prescribed according to subjective refraction meet withgreater patient satisfaction than those that would be prescribedaccording to autorefractors. Those patients that had more than an 0.5Diopter discrepancy will almost always be unhappy with eyeglassesprescribed according to the autorefraction, and happy with theeyeglasses prescribed according to subjective refraction.

Accordingly, it is standard practice in the evaluation of autorefractorsto consider subjective refraction to be the “gold standard” since itcorrelates better with patient visual experience than any othermeasurement. Consequently it is standard practice for ophthalmologiststo fine-tune the autorefraction values by performing a subjectiverefraction on the patient using a phoropter. The result is thatautorefractors are only used for screening purposes, or for giving anoptometrist or ophthalmologist a good starting point in doing asubjective refraction.

Objective refractors use infrared light to measure the eye becauseinfrared light reflects much more strongly out of the eye than visiblelight does. However, there are fundamental physical reasons to expectproblems with the approach of using an infrared light beam toautorefract a patient or subject.

First, when an objective refraction measurement is performed, the lightis scattered back to a measurement device from a location in the eyethat is not the same as the location of the photoreceptors in the eye.That means that the above-described autorefraction measurement isreferenced to a location that does not correspond to where photons arebeing converted into neural impulses. It has been theorized that theinfrared light scatters off the choroid in the above-describedautorefraction measurement, while others theorize that the light passesthrough the choroid and scatters off of the sclera. Regardless of theexact location at which the light scatters (hereinafter referred to as“the scattering location”), there is a definite distance between thescattering location and the photoreceptors. This phenomenon may beunderstood with reference to FIG. 5, which shows an infrared probe beam25 passing into the eye 500 through the cornea 540 and lens 530, passingthrough the fovea 520, and striking the choroid 510. In FIG. 5, it isshown that the light scatters off the choroid 510 instead of thephotoreceptors, although as explained above, the principle appliesregardless of the actual scattering location.

Second, the eye works at visible wavelengths, but the infrared radiationis subject to chromatic aberration and that changes the refractionvalues. Fortunately, many papers have been published on the effects ofchromatic aberration in the human eye. It is relatively easy to use thepublished data to make accurate adjustments to the refractioncalculation based on published chromatic aberration values.

It is hypothesized that the significant structural difference in the 20%of patients with significant differences between subjective refractionand autorefraction is that the spacing between the photoreceptors to thescattering location is different in those patients relative to thegeneral population. However the literature on the spacing or distancebetween the photoreceptors and the scattering location is much moresparse, and does not extend much past a few brief paragraphs in journalsand textbooks. The effect of the chromatic aberration is such that theraw measurement will measure incorrectly by about negative 1.4 Diopters.The effect of the spacing or distance between the photoreceptors and thescattering location is such that the raw autorefractor value will be inerror by about a positive 0.8 Diopters. The combined effect of the twoadjustments is that the raw measurement from the autorefractor needs tobe adjusted by about negative 0.6 diopters in order to agree with thesubjective refraction.

Meanwhile, the Stiles-Crawford effect is also suggested as beingresponsible for causing discrepancies between subjective refractions andautorefractions. The Stiles-Crawford effect refers to the fact the conesin the eye show a marked preference to respond to light that is within arelatively narrow range of angles. (The fovea is where high resolutionvision occurs and it is packed very densely with cones.) The effect issuch that a ray of light entering the edge of a 7 mm pupil will cause aresponse that is about 22% as strong as a ray that enters the center ofthe pupil.

In normal eyes, the photoreceptors are pointed so that the peak responseis pointed to somewhere in the center 1.0 mm region of the pupil.However, it can happen that the cones point toward the edge of thepupil.

One hypothesis is that the autorefractors measure inaccurately becausethey calculate the sphere cylinder and axis paying special attention toweight the light in the center of the pupil the most strongly. Butconsider the possibility that the eye is really weighting the lower halfof the pupil more heavily than the center. If the sphere value in thatregion of the eye is one diopter different than it is in the center ofthe pupil, the autorefractor would read incorrectly by one diopter.Wavefront aberrometry measurements on patients can indicate how much ofa difference might be caused in a subjective refraction due to aStiles-Crawford effect.

Objective and subjective methods have been developed to evaluate thestrength of the Stiles-Crawford effect, and to locate the position onthe eye's pupil that is weighted the most strongly in vision. Theknowledge of that location, along with a refractive power map derivedfrom a wavefront aberration map, can be used to calculate improvedvalues of sphere cylinder and axis that would better correspond to thosethat would be obtained by a subjective refraction.

One would expect that only subjects with large high order aberrationswould be affected by the Stiles-Crawford effect. However, a number ofsubjects have been measured that have significant differences betweensubjective and objective refractions and they had very small high orderaberrations. This observation supports the view that variations in thedistance between the scattering layer and the photoreceptors is aprimary reason for differences in objective and subjective refractionsin those subjects, although the Stiles-Crawford effect still may play arole in some subjects.

Another factor that affects the accuracy of an autorefraction is thedistance from the eye to the instrument. Autorefractors typicallycontain a method to assist the doctor in setting this distance to theoptimal value. Many other instruments such as corneal topographerscontain very accurate methods of setting that distance. One of thesimplest being a camera that looks at the head from the side so that thecornea is seen in profile and the instrument moved back and forth untilthe apex of the cornea lines up with a reticle on a video screen.

Accordingly, it would be desirable to provide a method and system tomeasure the spacing or distance between the photoreceptors and thescattering location during an objective refraction measurement, and amethod to use that parameter to improve the calculated sphericalequivalent power on those patients. It would also be desirable toprovide an instrument including an optical beampath for testing thehypothesis that the Stiles Crawford effect is responsible for thedifference in autorefractions versus subjective refractions.

The present invention comprises a system and method for measuring adistance between the photoreceptors and the scattering location in aneye. Beneficially an objective refractor is employed to perform anobjective refraction of the eye and to measure the distance between thephotoreceptors and the scattering location in an eye. The objectiverefractor could be an autorefractor, a wavefront aberrometer, aphotoretinoscope, or a similar device that relies on objectivelymeasuring the eye.

In another aspect of the invention, a method for measuring a distancebetween the photoreceptors and the scattering location in an eyecomprises performing an autorefraction of the eye with an objectiverefractor; focusing the eye on a rotating speckled light pattern;adjusting a distance between the speckled light pattern and the eyeuntil the speckled light pattern appears to be stationary; measuring thedistance between the speckled light pattern and the eye when thespeckled light pattern appears to be stationary; and calculating thedistance between the photoreceptors and the scattering location based onthe distance between the speckled light pattern and the eye.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a functional diagram of a wavefront aberrometer;

FIG. 2 shows the results of clinical trials comparing subjectiverefraction measurements with refractive measurements produced by awavefront aberrometer;

FIG. 3 shows an optical set-up to observe a rotating speckle pattern;

FIG. 4 shows a functional diagram of a modified wavefront aberrometer;

FIG. 5 illustrates several pertinent elements of a human eye; and

FIG. 6 illustrates a device for correcting out subject astigmatism whileobserving a speckle pattern on a spinning disk.

DETAILED DESCRIPTION

FIG. 1 shows a functional diagram of a wavefront aberrometer 100. Thewavefront aberrometer 100 is a commercially successful instrument thathas been used primarily by ophthalmologists for making fine adjustmentsto refractive laser eye surgery nomograms for treatment of myopia andastigmatism using the Lasik procedure. The wavefront aberrometer 100 canbe thought of as a super-autorefractor that performs all the functionsof a regular autorefractor but adds the capability to measure high orderaberrations of the eye. The wavefront aberrometer 100 does this bybreaking the eye into a grid with a spatial resolution of 0.2 mm spacingand measuring the optical performance of each zone. A description of awavefront aberrometer such as the wavefront aberrometer 100 can be foundin U.S. Pat. No. 6,550,917 issued on 22 Apr. 2003 in the names of DanielR. Neal, Darrel J. Armstrong, Daniel M. Topa, and Richard J, Copland,the entirety of which is hereby incorporated herein by reference for allpurposes as if fully set forth herein.

It should be understood that in lieu of the wavefront aberrometer 100,an autorefractor, a wavefront aberrometer, a photoretinoscope, oranother type of objective refractor that performs objective measurementson the eye could be employed in the system and methods described below.

In the wavefront aberrometer 100, an infrared SLD beam is injected intothe eye. A stage inside the wavefront aberrometer 100 moves so that theconvergence of the SLD beam entering the eye is such that a small spotfocuses on the retina. The subject is instructed to look at the targetinside the wavefront aberrometer 100 so that the spot focuses on thefovea centralis. Light scatters in all directions from the scatteringlocation. Some of the light scatters back through the pupil of the eye.If the lens and cornea of the eye were perfect, all the light raysexiting the pupil would be parallel and the wavefront would be planar.

Of course a real eye is not perfect so that the rays exiting the pupilare not all parallel. The wavefront sensor inside the wavefrontaberrometer 100 is located at a plane that is conjugate to the cornea sothat it measures the deviations from parallel of rays as they leavewell-defined regions of the cornea.

The optical layout of the wavefront aberrometer 100 provides animportant advantage over other similarly designed aberrometers. Thewavefront sensor, collimated SLD beam and the fixation target that thesubject looks at are all located on the moving stage. The result is thewhen the stage moves to focus the infrared beam on the retina, thewavefront sensor and the fixation target automatically move into thecorrect position for a good measurement. The arrangement also works wellfor the control of subject accommodation. Generally the stage isinitialized in a position where it is in the myopic region, and thestage moves toward the hyperopic region. During this motion, the eyetarget will temporarily appear clear to the subject, but at that timethe spot on the retinal camera will appear fuzzy. The stageautomatically continues to move toward hyperopia until the SLD spotbecomes well focused. At that point, the target appears fuzzy so as tokeep the subjects focused as close to infinity as the eye is capable of.

A simple eye model has been developed to convert measurements from thewavefront aberrometer 100 to spherical equivalent values of a human eye.The eye model places all the refractive power in a spherical surface atthe cornea, and fills the space between the “cornea” and the “fovea”with a dispersive substance that has index of refractions matching thosepublished by Dr. Larry Thibos of the University of Indiana (e.g., LarryN. Thibos, et al., “The Chromatic Eye: a New Model of Ocular ChromaticAberration,” APPLIED OPTICS 31, 1992, 3594-3600).

Ray tracing analysis and experimental data have shown the basic validityof the equation shown below.S _(vis) =P _(eff)−(V _(ir)−1/(L+D))*(n _(vis)−1)/(n _(ir)−1),where  (1)

S_(vis) is the spherical equivalent power at the cornea that we wish toknow;

V_(ir) is the quantity that the wavefront aberrometer actually measures,the radius of curvature of the infrared light coming out of the cornea;

P_(eff) is the effective power of the cornea and lens of the eye and isassumed to be 60;

L is the length of the eye that would result in a person having perfectvision for an assumed P_(eff) (It is equal to 1/Peff. For instance, a 60diopter cornea/lens combination would result in perfect vision if thephotoreceptors were 16.6666 mm away);

D is the distance between the photoreceptors in the eye and locationwhere the infrared light scatters (“the scattering location”);

-   -   n_(vis) is the average refractive index of the eye for visible        light and is 1.3343 at 550 nm; and

n_(ir) is the average refractive index of the eye for infrared light andis 1.3247 at 840 nm.

From ophthalmology references, it is known that P_(eff) ranges between57 and 63 diopters in humans. For simplicity, it can be assumed to be60. Performing a sensitivity analysis of the terms in the formula above,it is seen that with 63 in the equation above, the effect on the S_(vis)value is only about 0.02 diopters since the relation L=1/P_(eff) iscontained in the equation for S_(vis).

The parameters n_(vis) and n_(ir) are obtained from an equation that Dr.Larry Thibos published based on experimental measurements. The values heobtained are average values. Thibos' data did not extend to 840 nm, butthe curve in the IR was almost flat and was very smooth so that it isappropriate to extrapolate the curve.

It is standard practice for D to be assumed to be a constant value. Avalue between 0.14 and 0.25 is probably used by most autorefractorsoftware. The value is usually determined experimentally by choosing thevalue that gives the best fit between many subjective refractions andthe autorefractor measurements. The calculated S_(vis) value is verysensitive to the exact value of D. A change from 0.125 mm to 0.250 mmchanges the spherical equivalent value by 0.4 Diopters.

Since the effect of the chromatic correction is about twice that due tothe distance between the photoreceptors and the scattering location, itwould make sense to suspect that inaccurate refractions are more relatedto chromatic aberration. However, analysis of data from clinical trialshas indicated that applying a fixed chromatic adjustment works equallywell among many different subjects. The reasoning is this: according tooptical ray tracing analysis, if the refractive errors were due to animperfect chromatic adjustment, the magnitude of the errors (expressedin diopters) should increase for subjects that are more myopic. However,that trend is not seen. Instead the range of the refraction errors is nolarger for strongly myopic subjects than it is for emmetropic subjects.

That leaves the distance between the photoreceptors and the scatteringlocation as the remaining parameter to study. According to optical raytracing, a variation of about 0.1 mm in the distance between thephotoreceptors and the scattering location would result in the sameshift in the refraction error as if the subject were strongly myopic oremmetropic. In fact, this is the behavior that has been seen in clinicaltrials, as shown in FIG. 2.

The wavefront aberrometer 100 is designed to measure high orderaberrations of the eye, which it does very well. However, it can alsomeasure the sphere, cylinder and axis like an autorefractor.

Many ophthalmologists have observed that the wavefront aberrometer 100obtains the same value that they get when they do a subjectiverefraction on themselves. However, there are a number of otherophthalmologists that have observed that the wavefront aberrometer 100disagrees by as much as one diopter from the sphere value that they geton a subjective refraction. Some of these ophthalmologists have observedthat autorefractors also always get the wrong sphere value on them.

Like most autorefractors, the wavefront aberrometer 100 also uses aninfrared probe beam to perform the measurement so it is likely that thewavefront aberrometer 100 and autorefractors share a systematic bias onsome subjects.

Frequently the ophthalmologist will express frustration thatautorefractor accuracies are specified based on the averages over manypatients. They note that a machine that is wrong on one out of fivepatients is nearly useless to them, regardless of how good the averagevalue of a hundred patients is.

A recent clinical trial was performed on 20 subjects (40 eyes) tocompare subjective refractions to autorefractions obtained using thewavefront aberrometer 100 and a NIDEK® ARK-2000 autorefractor. Theresults were consistent with the hypothesis that the distance betweenthe photoreceptors and the scattering location ranges between 0.1 and0.4 mm, and that range accounts for the disagreements in subjectiverefractions and autorefractions.

Further, the wavefront aberrometer 100 and NIDEK® instruments obtainedsimilar measurements in those cases where the autorefractions disagreedwith the subjective refractions.

The wavefront aberrometer 100 and NIDEK® instruments are different inthe optical principles that they use to measure the refractions. Howeverthey are similar in that they both use an infrared probe beam. Thisfurther supports the hypothesis that the main reason for discrepanciesbetween autorefractions and subjective refractions is that the infraredprobe beam scatters at a different location than where thephotoreceptors are.

Accordingly, it is desired to modify the wavefront aberrometer 100 ofFIG. 1 to account for the actual location of the photoreceptors in aneye.

There are three difficulties with attempting to measure the location ofthe photoreceptors. The most obvious difficulty is that thephotoreceptors are strongly absorbing of visible light. It is difficultfor an external instrument to get a sufficiently strong reflection tolocate the photoreceptors.

The second difficulty is that between the photoreceptors and the choroidthere is a layer of tissue named the retinal pigment epithelium (RPE).This layer is also strongly absorbing of visible light. Its purpose isto prevent reflected visible light from scattering back into the foveaand reducing visual acuity.

The third difficulty is that the photoreceptors are transparent toinfrared light. This fact was implicit in the earlier discussions thatdescribed how autorefractors work by reflecting light off of the choroid(or sclera) behind the photoreceptor layer.

Despite these difficulties, there is a relatively simple method tolocate the photoreceptor layer. That is, the photoreceptor layer may belocated by giving the subject a control that he can adjust until he seessome particular phenomenon occur inside the wavefront aberrometer. Thishas the advantage over other methods in that it involves the subject'sphotoreceptors in the way that they are actually used. With properdesign, the task that the subject would be presented with would be muchsimpler than making a subjective judgement, such as judging if aprojected letter is fuzzier or clearer.

The simplest task that the subject could perform would be to align twodots that he sees. In that case, the subject adjusts the convergenceangle between two narrow beams until they overlap the same region of thephotoreceptor layer. The adjustment has to be calibrated against theother optics in the autorefractor to make the result meaningful.

A typical embodiment of such a scheme is a Scheiner disk. It comprises amask with two holes that is placed near a lens. A beam of light shinesthrough the lens. At the image plane behind the lens, the beams from thetwo holes will overlap. The distance between the dots increases thefarther an observation plane is moved farther from the focal plane.

Another task that a subject can perform, that is relatively easy toimplement in hardware, is to adjust the apparent motion of a specklepattern on a rotating speckle wheel or spinning disk.

The phenomenon of laser speckle may be unfamiliar to non-specialist inoptics. (It almost certainly will be unfamiliar to patients.) Speckle isa very striking phenomenon of laser light. When a person looks at alaser beam that is illuminating a diffuse surface, such as paper orground glass, the person will see a random collection of bright and darkspots. This phenomenon results because the wavelength of light is muchsmaller than the rough features on a diffuse surface. The surface actslike a sheet of randomly distributed small scattering features. Thegranular appearance results from constructive and destructiveinterference of light waves that occurs on the observer's retina.Speckle patterns can be observed if one shines a laser pointer pen at ablack piece of paper. (If the laser hits a white piece of paper, thereflection is so bright that it is hard to see anything except a blur.But if you shine the laser pen across the room so the beam spreads outsome, you will see the speckle patterns if you look where the beam hitsthe wall).

Speckle patterns can be used to locate the focal plane of a lens. Atypical method is illustrated with respect to FIG. 3. The camera has alens that is focused at infinity. The camera looks through the lens atthe glass disc and the laser beam illuminates the glass disc from off tothe side.

When the glass disc is too close to the lens, the speckle pattern willappear to a viewer to have an apparent motion in one direction. If theglass disc is too far from the lens, the apparent motion will be in theopposite direction. The speed of the apparent motion increases thefarther away from focus the glass disc is located. For the special casethat occurs when the glass disc is exactly at the focal point of thelens, the speckle pattern will appear not to have any net motion.Instead, the dots will appear to randomly oscillate between bright anddark while swimming around in a random manner.

Accordingly, a subject is given the task to turn a knob that changes howfar the spinning glass disc, or rotating speckle wheel, is located fromthe lens, until it appears to the subject that the speckle pattern isstationary. In the diagram shown in FIG. 3, the CCD sensor chipcorresponds to the subject's retina and the camera lens corresponds tothe combined lens and cornea optical elements of the eye.

FIG. 4 shows a modified wavefront aberrometer 400 with an additionalbeam.

A laser used to generate the speckle pattern is beneficially red sincethat color tends not to stimulate the visual accommodative response. Atthe same time that the subject is viewing the speckle pattern, he alsosees a white crosshair pattern that is slightly fogged so that infinityfocus is maintained. The speckle pattern may be continuouslyilluminated, or it may flash, or pulse, on and off periodically. If ared wavelength is used, the chromatic aberration of the eye will have tobe considered in the calculation of the distance between thephotoreceptors and the scattering location. However, if the samewavelength is used in both the wavefront sensor path and the subjectivepath, the calculation is more accurate as it does not include acorrection for the chromatic aberration. In a particularly usefulwavefront sensor according to in U.S. Pat. No. 6,550,917 as referencedabove, the measurement wavelength is 840 nm, which the eye sees as a dimred color. Illumination of the speckle wheel (disk), or the Scheinerdisk, can also be done at the 840 nm wavelength, but the system has tobe designed so that the illumination is bright enough but is also stillsafe. Pulsed operation of the light source can be useful in reducing theoverall light energy deposited into the eye while maintaining thesubject's ability to see well enough to provide subjective input.

Speckle optometry has been used to measure the refraction of the eye. Inmost cases, it has been desired to measure the refractive state of theeye when it is focused at its far point. Such systems and methodstypically have the subject view a target across the room through a beamsplitter and a reflected image of the speckle wheel (spinning disk),that flashes on periodically. This keeps the eye focused at the farpoint and not at some nearer distance. A similar system may be includedin the measurement of the distance between the photoreceptors and thescattering location, in that a target stimulus may appear at the farpoint, or at some nearer position that stimulates accommodation.However, it is not necessary to include a target stimulus for thecalculation of the distance between the photoreceptors and thescattering location, since that parameter does not change depending onthe accommodation of the eye. The subject can provide the subjectiveinput by manipulating the Scheiner disk or the rotating speckle wheel(spinning disk) at any accommodative state, including an empty field,and then the objective refraction measurement can be madesimultaneously, and the distance between the photoreceptors and thescattering location can be calculated from the results. Then that valuecan be used to improve the measurement of the objective refraction thatis made at a different time, when the eye focused at it far point.

The accuracy of the calculation for improving the refraction measurementdepends on how accurately the subject can provide the subjective inputon when the speckle pattern appears to be stationary. The reportedaccuracy and repeatability of speckle optometry measurements rangesbetween 0.2 and 0.5 diopters. One factor that reduces the repeatabilityof the speckle optometry is that the eye can have astigmatism. Thisincreases the depth of focus where the speckle pattern may appearstationary.

There are several methods that can be used to decrease the depth offocus so that a more repeatable measurement can be obtained. One methodis to introduce a lens that corrects for the astigmatism of the eye.This lens can be chosen based on the objective measurement of theastigmatism, and it may be put in place automatically in the opticaltrain.

Another method that will reduce the depth of focus is to align themotion of the rotating speckle wheel (spinning disk) according to theaxis of astigmatism that has been determined by the objectivemeasurement, and this also can be done either automatically or manually.FIG. 6 illustrates a device 600 for correcting out subject astigmatismwhile observing a speckle pattern on a disk 610 which is being rotatedby the spinning motor 620. The arrangement of FIG. 6 shows how thedirection of the apparent motion of the speckles can be varied tocorrect for a subject's astigmatism. When the stepper motor 630 movesthe disk 610 to a new stationary location, the direction of motion onthe spinning disk 610 will appear to have changed to the subject. Asimilar arrangement for a gimbaled drum has been shown by otherresearchers (e.g., Henry A. Knoll, “Measuring Ametropia with a GasLaser,” American Journal of Optometry and Archives of American Academyof Optometry, July 1966, volume 43, number 7, page 415-418).

Still another method to decrease the depth of focus is called the“Method of Limits.” Instead of the subject adjusting the location of thedisc for stationarity, the subject adjusts the location until thespeckle motion is just barely observable to move in one direction andthat location is recorded. Then the subject adjusts the location of therotating speckle wheel (spinning disk) until the speckle motion is justbarely visible in the opposite direction and that location is recorded.Then the location for best stationarity is calculated as halfway inbetween the two recorded positions.

Yet, another method has been developed that converts the speckles intostreaming lines using a spinning prism. (Hitoshi Ohzu, “The applicationof lasers in ophtalmology and vision research,” Optica Acta, volume 26,number 8, page 1089-1101, 1979).

Spherical aberration is another factor that will increase the depth offocus where the speckle motion appears stationary. One way to decreasethat effect is to optically compensate the speckle optical path for thespherical aberration of an average human eye, or to use adaptive opticsto compensate for the spherical aberration of the eye that is beingtested. Another method is to optically project the speckle illuminationinto the eye in a manner that only illuminates some small region ofentrance into the eye. Such projection systems are sometimes used infundus imaging cameras to improve the image quality by avoiding regionsof the cornea that have more aberrations and these systems image anaperture stop at or near the exit pupil of the eye.

An advantage of the speckle method is that the location of the glassdisc can be measured very accurately with electronic micrometers, andits location can be related mathematically by ray tracing to thelocation of the photoreceptors relative to the scattering location.

One of the difficulties in making any objective refraction measurementis that there are constant fluctuations in the accommodative focus ofthe human eye on the order of 0.5 diopters. Some researchers havetheorized that these fluctuations are part of the mechanism that helpsthe eye maintain proper focus. Other researchers have noticed that somecomponent of the fluctuations corresponds to the subject's heartbeat inabout half of the population. The impact of the heartbeat on eye'saccommodative response is unclear. However, it is clear that if anobjective refraction were made at an instant that the focus isfluctuating, the measurement would disagree with a subjective refractionby the amount of the fluctuation. Several researchers have measuredthese fluctuations with special autorefractors that provide a timeseries analysis of the eyes accommodation.

Accordingly, it is important that the wavefront aberrometer describedabove makes its measurements of the eye at the same time that thesubject is making the judgment that the speckle pattern is stationary,or is barely moving. This can be accomplished by electronicallysynchronizing the measurements made by the wavefront aberrometer with aflashing light source (e.g., a pulsed laser) that illuminates therotating speckle wheel (spinning disk), or with flashing light (e.g., apulsed laser) that illuminates the Scheiner disk.

A drawback of the speckle method is that is has been tried before in themeasurement of chromatic aberration. In that application, the deducedrefractive index measurements of the eye versus wavelength disagreedwith a number of different techniques of measurement of chromaticaberration of the eye that all showed fairly good agreement with eachother. To confirm that the speckle method works as expected in findingthe distance between the photoreceptors and the scattering location, aconventional method for measuring the chromatic aberration is alsoprovided.

There is a subtle point concerning speckle that may have been overlookedin previous studies. According to diffraction theory, the specklepatterns are generated because light from many different directions isconverging onto a photodetector. However, the Stiles-Crawford effectshows that the cones in the eye have a waveguide nature that effectivelygives each cone a strongly preferred direction of light to which it willrespond. The cones are only about 0.070 mm in length, so the distancefrom the photopigments to the front of the cones is not expected to haveany significant effect on the effect on the measurement of the distancebetween the photoreceptors and the scattering location, but the conedirectionality might. Consequently, it is important that the specklegenerating system be properly designed to match the acceptance angles ofthe cones. The laser coherence length, the surface roughness of thespinning disk, the disk velocity, the focal length of the intermediatelens and its numerical aperture are the major variables. An adjustableiris is included at the intermediate lens in order to make the numericalaperture of the lens adjustable.

The light source for performing the chromatic aberration measurement isa white light source that has a filter wheel with 50 nm wide filters.The filtered light is coupled into an optical fiber, and the light outof the fiber is collimated by an achromatic lens. Software in thewavefront aberrometer 400 is able to move the stage until the filteredlight appears to be the smallest size it can reach as it appears on asecond retina camera (CCD3). The lens in front of that retina camera iscompensated for the known chromatic aberrations of the eye. For thisapplication, the compensation can be approximate since only relativeretinal spot sizes are needed.

The chromatic aberration of the eye is well known, and the provision fora chromatic aberration measurement is to provide an additionaldiagnostic tool in the event that the speckle methodology holds someunexpected surprises.

In operation, autorefraction is performed as usual with the wavefrontaberrometer 400, except that the subject is given an additional task.After the standard autorefraction is completed, the subject continues tolook at the fogged target and a red laser is turned on to illuminate thespinning glass disk. The subject adjusts a knob until the specklepattern appears to be stationary. Then the wavefront aberrometer 400software records the location of the spinning glass disc when thesubject indicates that it produces the stationary speckle pattern. Thelocation information may be provided in terms of: (1) a distance betweenthe speckled light pattern (or Scheiner disk) and the eye when thespeckled light pattern appears to be stationary to the subject (or thetwo dots overlap in the case of the Scheiner disk), or (2) a distancethat the speckled light pattern (or Scheiner disk) was moved from anominal position, to the position where the speckled light patternappears to be stationary to the subject (or the two dots overlap in thecase of the Scheiner disk).

Using the well-known thin lens equation, the location information isused to calculate the distance between the photoreceptors and thescattering location, and the value used in the sphere equation that thewavefront aberrometer 400 uses when calculating the sphere, cylinder andaxis for a subject.

The formula to calculate the correction to the sphere value is:dP=f^2/x1−f^2/x2, where  (2)

dP is the correction to the sphere value;

-   -   f is the focal length of the lens that is in between the eye and        the rotating speckle wheel (spinning disk);

x1 is the distance from the lens to the rotating speckle wheel (spinningdisk) for a stationary appearance of the speckle pattern for an eye thathas the nominal distance between the photoreceptors and the scatteringlayer; and

x2 is the distance from the lens to the rotating speckle wheel (spinningdisk) for a stationary appearance of the speckle pattern for the eyethat is being measured.

A comparison is made to determine if the use of the “customized”distance between the photoreceptors and the scattering location gives abetter match of wavefront aberrometer 400 refraction to subjectiverefraction than the use of the average distance between thephotoreceptors and the scattering location. The location of the spinningdisk when the speckle pattern is stationary can be used directly tocalculate the adjustment to the autorefracted sphere value. Thecalculation of the distance between the photoreceptors and thescattering location is an additional data reduction step that does nothave to be performed if the user is only interested in the value of thesphere adjustment.

When the customized data give better results, it shows that the distancebetween the photoreceptors and the scattering location has beenmeasured.

While preferred embodiments are disclosed herein, many variations arepossible which remain within the concept and scope of the invention.Such variations would become clear to one of ordinary skill in the artafter inspection of the specification, drawings and claims herein. Theinvention therefore is not to be restricted except within the spirit andscope of the appended claims.

1. A method for improving the calculation of sphere and cylinder in anautorefraction by determining a distance between photoreceptors and ascattering location in a subject's eye, comprising: performing anautorefraction of the eye with an objective refractor to calculate asphere and cylinder value for the eye; focusing the eye on a rotatingspeckled light pattern; adjusting a distance between the speckled lightpattern and the eye until the speckled light pattern appears to bestationary; measuring the distance between the speckled light patternand the eye when the speckled light pattern appears to the subject to bestationary; calculating the distance between the photoreceptors and thescattering location based on the distance between the speckled lightpattern and the eye; and adjusting the sphere and cylinder calculationto include the calculated distance between the photoreceptors and thescattering location.
 2. The method of claim 1, wherein theautorefraction is performed with a wavefront aberrometer.
 3. The methodof claim 2, wherein the wavefront aberrometer includes an SLD lightsource, a fixation target, and a wavefront sensor all located on amovable platform, and wherein performing the autorefraction includesmoving a position of the stage with respect to the eye.
 4. The method ofclaim 1, further comprising producing the rotating speckled lightpattern by illuminating a spinning disk with a first light source. 5.The method of claim 4, wherein the objective refractor performs theautorefraction using a second light source, and wherein the first andsecond light sources emit light having about a same wavelength.
 6. Themethod of claim 4, wherein the first light source emits red light. 7.The method of claim 4, where the first light source produces illuminatesthe spinning disk with flashing light.
 8. The method of claim 1, whereinan astigmatism of the eye is corrected for while focusing the eye on thespeckled light pattern.
 9. The method of claim 8, wherein theastigmatism is corrected by introducing a lens into an optical pathbetween the eye and the speckled light pattern, wherein the lens isselected based on an astigmatism value determined by the autorefraction.10. The method of claim 8, wherein the astigmatism is corrected byaligning a motion of the speckled light pattern to an axis of theastigmatism, where the axis of the astigmatism is determined by theautorefraction.
 11. The method of claim 1, wherein a sphericalaberration of the eye is at least partially corrected for while focusingthe eye on the speckled light pattern.
 12. The method of claim 1,further comprising measuring a chromatic aberration of the eye.
 13. Themethod of claim 1, wherein the subject focuses on a target object whileadjusting a distance between the speckled light pattern and the eyeuntil the speckled light pattern appears to be stationary.
 14. Themethod of claim 1, wherein the subject does not focus on any targetobject while adjusting a distance between the speckled light pattern andthe eye until the speckled light pattern appears to be stationary.
 15. Amethod for improving a measurement of sphere and cylinder in anautorefraction by determining the chromatic aberration of a subject'seye, comprising: performing an autorefraction of the eye with anobjective refractor to calculate a sphere and cylinder value for theeye; focusing the eye on a rotating speckled light pattern; adjusting adistance between the speckled light pattern and the eye until thespeckled light pattern appears to the subject to be stationary whenlight having a first wavelength, approximately equal to the wavelengthof an infrared probe beam of the objective refractor, is used toilluminate the spinning disk; adjusting a distance between the speckledlight pattern and the eye until the speckled light pattern appears to bestationary to the subject when light having a second, visible wavelengthis used to illuminate the spinning disk; measuring a difference in thelocations of the spinning disk at the two wavelengths and calculatingthe chromatic aberration; and adjusting the sphere and cylindercalculation to include the calculated chromatic aberration.
 16. Themethod of claim 15, wherein the autorefraction is performed with awavefront aberrometer.
 17. The method of claim 16, wherein the wavefrontaberrometer includes an SLD light source, a fixation target, and awavefront sensor all located on a movable platform, and whereinperforming the autorefraction includes moving a position of the stagewith respect to the eye.
 18. The method of claim 15, further comprisingproducing the rotating speckled light pattern by illuminating a spinningdisk.
 19. A method for improving the measurement of sphere and cylinderin an autorefraction by determining a distance between photoreceptorsand a scattering location of a subject's eye, the method comprising:performing an autorefraction of the eye with an objective refractor tocalculate a sphere and cylinder value for the eye; providing light froma first light source through two apertures of a Scheiner disk to projecttwo dots of light onto the subject's eye; adjusting a distance betweenthe first light source and the Scheiner disk until the two dots of lightfrom the Scheiner disk appear to the subject to overlap; measuring adistance from the first light source to the Scheiner disk when the twodots appear to the subject to overlap; calculating the distance betweenthe photoreceptors and the scattering location based on the distancebetween the first light source and the Scheiner disk when the two dotsappear to the subject to overlap; and adjusting the sphere and cylindercalculation to include the measurement of the distance between thephotoreceptors and the scattering location.
 20. The method of claim 19,wherein the autorefraction is performed with a wavefront aberrometer.21. The method of claim 20, wherein the wavefront aberrometer includesan SLD light source, a fixation target, and a wavefront sensor alllocated on a movable platform, and wherein performing the autorefractionincludes moving a position of the stage with respect to the eye.
 22. Themethod of claim 19, wherein the objective refractor performs theautorefraction using a second light source, and wherein the first andsecond light sources operate at about a same wavelength.
 23. The methodof claim 19, wherein an astigmatism of the eye is externally correctedfor while projecting the two dots of light onto the subject's eye. 24.The method of claim 23, wherein the astigmatism is corrected byintroducing a lens into an optical path between the eye and the speckledlight pattern, wherein the lens is selected based on an astigmatismvalue determined by the autorefraction.
 25. The method of claim 23,wherein the astigmatism is corrected by aligning a motion of thespeckled light pattern to an axis of the astigmatism, where the axis ofthe astigmatism is determined by the autorefraction.
 26. The method ofclaim 19, wherein a spherical aberration of the eye is at leastpartially corrected for while focusing the eye on the speckled lightpattern.
 27. The method of claim 19, further comprising measuring achromatic aberration of the eye.
 28. The method of claim 19, wherein thesubject focuses on a target object while adjusting a distance betweenthe speckled light pattern and the eye until the speckled light patternappears to be stationary.
 29. The method of claim 19, wherein thesubject does not focus on any target object while adjusting a distancebetween the speckled light pattern and the eye until the speckled lightpattern appears to be stationary.
 30. A method for improving thecalculation of sphere and cylinder in an autorefraction by determining adistance between photoreceptors and a scattering location in a subject'seye, comprising: performing an autorefraction of the eye with anobjective refractor to calculate a sphere and cylinder value for theeye; focusing the eye on a rotating speckled light pattern produced on aspinning disk; adjusting a distance between the speckled light patternand the eye by moving the spinning disk from a nominal position to afocused position where the speckled light pattern appears to the subjectto be stationary; measuring the distance between the nominal positionand the focused position where the speckled light pattern appears to thesubject to be stationary; and adjusting the sphere and cylindercalculation to include a shift in lens power that corresponds to themeasured distance between the nominal position and the focused position.31. The method of claim 30, wherein the autorefraction is performed witha wavefront aberrometer.
 32. The method of claim 31, wherein thewavefront aberrometer includes an SLD light source, a fixation target,and a wavefront sensor all located on a movable platform, and whereinperforming the autorefraction includes moving a position of the stagewith respect to the eye.
 33. The method of claim 30, further comprisingproducing the rotating speckled light pattern by illuminating thespinning disk with a first laser.
 34. The method of claim 33, whereinthe objective refractor performs the autorefraction using a secondlaser, and wherein the first and second laser operate at about a samewavelength.
 35. The method of claim 33, wherein the first laser emitsred light.
 36. The method of claim 30, wherein an astigmatism of the eyeis externally corrected for while focusing the eye on the rotatingspeckled light pattern.
 37. The method of claim 36, wherein theastigmatism is corrected by introducing a lens into an optical pathbetween the eye and the speckled light pattern, wherein the lens isselected based on an astigmatism value determined by the autorefraction.38. The method of claim 36, wherein the astigmatism is corrected byaligning a motion of the speckled light pattern to an axis of theastigmatism, where the axis of the astigmatism is determined by theautorefraction.
 39. The method of claim 30, wherein a sphericalaberration of the eye is at least partially corrected for while focusingthe eye on the speckled light pattern.
 40. The method of claim 30,further comprising measuring a chromatic aberration of the eye.
 41. Themethod of claim 40, wherein a spherical aberration of the eye is atleast partially corrected for while focusing the eye on the speckledlight pattern.
 42. A method for improving the measurement of sphere andcylinder in an autorefraction by determining a distance betweenphotoreceptors and a scattering location of a subject's eye, the methodcomprising: performing an autorefraction of the eye with an objectiverefractor to calculate a sphere and cylinder value for the eye;providing light from a first light source through two apertures of aScheiner disk to project two dots of light onto the subject's eye;adjusting a distance between the light source and the Scheiner disk bymoving the light source from a nominal position to a focused positionwhere the two dots appear to the subject to overlap; measuring thedistance between the nominal position and the focused position where thetwo dots appear to the subject to overlap; and adjusting the sphere andcylinder calculation to include a shift in lens power that correspondsto the measured distance between the nominal position and the focusedposition.
 43. The method of claim 42, wherein the autorefraction isperformed with a wavefront aberrometer.
 44. The method of claim 43,wherein the wavefront aberrometer includes an SLD light source, afixation target, and a wavefront sensor all located on a movableplatform, and wherein performing the autorefraction includes moving aposition of the stage with respect to the eye.
 45. The method of claim42, further comprising producing two dots with a first light source. 46.The method of claim 45, wherein the objective refractor performs theautorefraction using a second light source, and wherein the first andsecond light sources operate at about a same wavelength.
 47. The methodof claim 42, wherein an astigmatism of the eye is externally correctedfor while focusing the eye on the two dots of light.
 48. The method ofclaim 47, wherein the astigmatism is corrected by introducing a lensinto an optical path between the eye and the speckled light pattern,wherein the lens is selected based on an astigmatism value determined bythe autorefraction.
 49. The method of claim 47, wherein the astigmatismis corrected by aligning a motion of the speckled light pattern to anaxis of the astigmatism, where the axis of the astigmatism is determinedby the autorefraction.
 50. The method of claim 42, further comprisingmeasuring a chromatic aberration of the eye.